1. Field of the Invention
The present invention relates to magnetic contacts for use in intrusion detection systems, and more particularly, to a protective housing for enclosing a reed switch.
2. Description of the Related Art
Intrusion detection systems, such as those used in burglar alarms, typically include two basic components: an intrusion detector and an intrusion indicator. An intrusion detector is used to monitor a given situation and provide information needed in determining whether an abnormal condition exists, such as the presence of an intruder. An intrusion indicator analyzes information received from an intrusion detector and determines whether an intrusion has occurred. If an intrusion has occurred, then the intrusion indicator generates some type of warning signal, such as flashing lights, ringing bells, or in more modern systems, a digital signal transmitted via telephone lines to a security system computer.
One type of intrusion detector is a metallic tape that is placed across all door and door frames and window and window frames of a building. The tape has an electric current passing through it which causes the intrusion indicator to determine that no intrusion has occurred. Any intruder entering the building will break the tape which will interrupt the current. The intrusion indicator will interpret the current interruption as indicating that an intrusion has occurred.
Another type of intrusion detector is a magnetic contact. A magnetic contact often includes a dry-reed switch (or simply "reed switch"), and an external magnet. FIG. 1 illustrates a reed switch 10. The switch 10 consists of two thin, metallic strips (or "reeds") 12 and 14 that are hermetically sealed in a delicate glass tube-like body 16. The glass body 16 is filled with an inert gas. Each of the reeds 12 and 14 is connected to a respective connecting lead 18 and 20.
When an external magnet is brought near the switch 10, the magnet attracts one of the reeds 12 or 14, which then contacts the other reed 12 or 14. When the reeds 12 and 14 come into contact, the circuit which is connected to the connecting leads 18 and 20 closes.
When used as an intrusion detector, the reed switch 10 and external magnet operate in a manner similar to the metallic tape detector. The switch 10 is usually installed in a hole in the wooden frame above a door. The external magnet is mounted on the door at a location which is near the switch 10 when the door is closed. When the door is closed, the reeds make contact due to the presence of the magnet, and current passes through the switch 10. When the door is opened, such as by an intruder, the external magnet is moved away from the switch 10 which causes the reeds to separate, and thus, the current is interrupted.
Before insertion into a hole of a wooden door frame, the reed switch 10 is typically enclosed within a cylindrical plastic housing in order to protect its delicate glass body 16. The cylindrical plastic housing has one open end and is generally long enough to enclose the entire length of the switch, as well as a short portion of the wires which are coupled to the connecting leads.
The reed switch must be secured within the plastic housing so that it cannot be pulled out during installation. Securing the switch within the housing, however, presents more of a problem than appears at first blush. Because the reed switch is a magnetic device, it cannot be secured in the housing with any type of metallic fastener because the metal would interfere with the operation of the switch. Furthermore, the reed switch is a delicate and very tiny device, often having a glass body length of only 0.53 inches. Thus, it cannot be secured in the housing by means of plastic screws, bolts, rivets, or like plastic fasteners because these fasteners are too large and would destroy the switch.
Therefore, after the reed switch has been inserted into the plastic housing, it has traditionally been secured by filling the entire housing with a fillant, typically epoxy adhesive. Since the epoxy is not metallic, it does not interfere with the operation of the switch. Since the epoxy is initially in liquid form, it does not harm the delicate switch. The entire housing can be quickly and easily filled with the liquid epoxy; this is a particularly advantageous quality because it has always been thought that filling the entire housing provided the best and most secure method of holding the switch in the housing. After it is hardened, the epoxy surrounds the entire switch such that there is no room for movement.
While the above method initially works well in securing a reed switch, it has been found that after a switch and housing have been implanted in a wooden door frame for a period of time, the switch often malfunctions. The malfunction is normally due to the glass body of the switch becoming cracked or broken. A reed switch will not function with a cracked or broken glass body.
Another problem that has arisen with the reed switch 10 relates to the closeness with which an external magnet must be brought near the reed switch 10 in order to cause the reeds 12 and 14 to make contact. Normally, the distance between an external magnet and the reed switch 10 must be less than one inch for the reeds 12 and 14 to make contact. If an error is made during installation in the positioning of the reed switch 10, external magnet, or both, the external magnet may not be able to get close enough to the reed switch 10 for the reeds 12 and 14 to make contact. Such an error in installation would prevent the reed switch from functioning.
Thus, there has emerged a compelling need for a reed switch housing which will protect the glass body of a reed switch from becoming cracked or broken over long periods of time, and for a reed switch assembly that will function properly even when small positioning errors occur during installation.